The use of modulation in the field of telecommunications is well known. Modulation is used to vary an input signal, such as a periodic waveform, so that the waveform can then be used to transmit or convey a message. After receiving the input signal, the communication device modulates the input signal to produce a transmittable signal.
Typically, a communication device includes components to receive an analog or digital input signal and then modulates the signal either using linear or polar modulation. Linear modulation is also known as direct or Cartesian modulation. Most devices are capable of handling only one of the two modes of modulation and therefore certain devices are restricted to predetermined communication protocols.
Use of a solely linear modulation circuit in a communication device, such as the one shown in FIG. 1, is known in the art. The linear modulation circuit 10 includes a first input 12, denoted as I_data, and a second input 14, denoted as Q_data. The first input 12 is connected to a multiplier 16 which multiplies the input with the value (cos ωt) with ω representing the radio frequency at which the wireless communications is performed and t representing a specific moment in time.
The second input 14 is connected to a multiplier 18 and multiplied with the value (sin ωt). The products, or outputs, from the multipliers 16 and 18 are then added together via a summer 20 to produce the transmittable signal. The disadvantage of this type of circuitry is that if there is any amplification or gain after the summer 20, it has to be linear in order for the signal to be used by other device circuitry. This requires more power to be drawn for the overall linear modulation process. This is unwanted since there is a limited amount of power available within the device.
Use of a communication device having only polar modulation circuitry, such as that which is shown schematically in FIG. 2, also has its disadvantages. The polar modulation circuitry 30 comprises a pair of inputs 32 and 34 whereby the first input 32 is denoted by A(t) while the second input 34 is denoted as V(t). These values are related to the two inputs in FIG. 1, but in polar form whereby V(t) represents the phase information of θ(t), and A(t) is the amplitude information.
The second input 34 is passed through a summer 35 before being transmitted to a voltage controlled oscillator (VCO) 36, and then combined along with the first input 32 using an amplifier 38. The output of the amplifier 38 is then transmitted to an antenna which transmits the output to another communicating party.
An output of the VCO 36 is fed back to the summer 35 through a phase lock loop (PLL) 37 in order to control the phase difference between the two inputs 32 and 34 after the second input 34 passes the VCO 36. As is known, there is a delay which is introduced into the circuitry 30 while the VCO 36 is in operation and therefore it is difficult to time the arrival of the first 32 and second 34 inputs at the amplifier 38 since both are based on time. Therefore, the PLL 37 locks the phase at time equals 0. One disadvantage is that the bandwidth of the PLL 37 can be restricted by the bandwidth of V(t).
It is, therefore, desirable to provide a hybrid linear and polar modulation apparatus.